Literally, when somebody's brain has stopped functioning such that they will never regain conciousness, hence coloquially somebody who is idiotic, dumb or clueless beyond belief. Similar to "too stupid to live".

The Urban Dictionary T-Shirt

the link from reality and fantasy are broken. there is no pilot in the cockpit if you know what I mean. the results of too many drugs can do this to you. you can be born this way. whatever the reason, you state things out of your ass without a reference, and you don't know nor care how accurate your statement is.

After I took over 10 years of bong hits, I have been left virtually brain dead, leaving only the cartoon network as a compatable reference.

The Urban Dictionary T-Shirt

The literal death of the brain, brought about by severence of the spinal cord or any major brain damage. There is argument as to whether or not brain death constitutes real death as some minor signs of life may still be seen, though the patient typically can't live without a respirator after brain death.

The Urban Dictionary T-Shirt

A increadibly idiotic individual who hasn't a sustance for anything that is going on. He or she is stupid, retarded, mental, ignorant, annoying, funny to make fun of, or any combination of the previous terms.